Modern telephone and data networks require light sources that are compatible with the silica fiber now almost universally used as the transmission medium of such networks. In the recent past, efforts were concentrated on lasers having an emission wavelength near at 1.55 .mu.m because of the minimum of optical absorption in silica at that wavelength. However, for very high-speed operation, particularly over relatively shorter distances, operation at 1.3 .mu.m is desirable because the dispersion in silica goes through zero at that wavelength. Dispersion determines the rate of change of, among other quantities, the speed of propagation of light on the fiber as a function of wavelength. A low dispersion allows a very high digital data rate. Therefore, modern communication networks require light sources that emit at 1.3 .mu.m, are inexpensive, and are robust.
It is generally believed that only diode lasers can fulfill the strict requirements placed on such light sources. A generic diode laser 2 is illustrated in FIG. 1. An active region 4 is sandwiched between p-type and n-type layers 6 and 7 which act both as the two sides of a diode and also as cladding layers that guide light parallel to and generally within the active region 4. When a power source 8 forward biases the diode 2 above its threshold current, the diode 2 lases and outputs light 9.
In U.S. patent application, Ser. No. 08/039,771, filed Mar. 30, 1993, now U.S. Pat. No. 5,381,434, which is incorporated herein by reference, we describe such a 1.3-.mu.m diode laser based on the AlGaInAs material system. Zah et al. describe further details of this work in "High-Performance Uncooled 1.3-.mu.m Al.sub.x Ga.sub.y In.sub.1-x-y /InP Strained-Layer Quantum-Well Lasers for Subscriber Loop Applications," IEEE Journal of Quantum Electronics, vol. 30, no. 2, February, 1994, also incorporated by reference. In particular, the diode we developed before includes multiple quantum wells that are composed of AlGaInAs and that are biaxially compressively strained.
Such compressively strained AlGaInAs diode lasers offer many advantages, including operation at the elevated temperatures expected in the field, and their reliability appears to be adequate. However, strained materials have often been suspect because strain is a possible failure mechanism causing failures which could be detected after only long periods of time. Accordingly, it is advisable to investigate other possibilities than compressively strained AlGaInAs.
Bhat et al. have previously reported a preliminary version of a tensile-strain AlGaInAs quantum-well diode laser in "OMCVD growth of strained Al.sub.x Ga.sub.y In.sub.1-x-y As for low threshold 1.3 .mu.m and 1.55 .mu.m quantum well lasers," Proceedings of the Fourth International Conference on Indium Phosphide and Related Materials, Apr. 21-24, 1992, Newport, R.I., paper THD2, pp. 453-456. This diode laser had a single quantum well of 14.4-nm thickness. Its temperature response was not disclosed. It is now believed that the AlGaInAs quantum wells described in that paper contained excessive Al or were too wide to adequately confine the electrons sufficient for high-temperature operation. Further, we believe that their single quantum wells of AlGaInAs emitting at 1.3 and 1.55 .mu.m insufficiently confined the electron carriers, had the wrong composition for temperature insensitivity, and had insufficient gain.
Two papers have described related lasers for use at 1.5 .mu.m. Kasukawa et al. disclose a diode laser having a single tensile-strain GaInAs quantum well and AlGaInAs barriers in "Very low threshold current density 1.5 .mu.m GaInAs/AlGaInAs graded-index separate-confinement-heterostructure strained quantum well laser diodes grown by organometallic vapor deposition." Applied Physics Letters, vol. 59, pp. 2486-2488, 1991. The tensile-strain laser had a characteristic temperature of 50.degree. K., which is considered too low for the high-temperature environment of the local telephone loop. Stegmuller et al. also disclose a strained diode laser having multiple compressively strained GaInAs quantum wells and AlGaInAs barriers in "1.57 .mu.m Strained-Layer Quantum-Well GaInAlAs Ridge-Waveguide Laser Diodes with High Temperature (130.degree. C.) Ultrahigh-Speed (17 GHz) Performance." in IEEE Photonics Technology Letters, vol. 5, pp. 597-598, 1993. However, their diode lasers like those of Stegmuller et al., insufficiently confined the electron carriers, excessively varied with temperature, especially with respect to the quantum efficiency and threshold current, and had insufficient gain. It is not clear how to adapt this laser design for 1.3 .mu.m. Their change in efficiency between 25.degree. and 85.degree. C. is large since they did not use the optimal barrier height and their barrier height does not conform to what we now believe to be the best.